Bacillus thuringienses (Bt) bacteria produce a natural insecticide. Widespread use of Bt has lead to Bt resistance among insects. Why is this occurring?

Some insects with Bt resistance genes are able to survive in the presence of Bt toxin. Natural selection favors these insects and increases their proportion in future generations.

Some insects with Bt resistance genes are able to survive in the presence of Bt toxin. Natural selection favors these insects and increases their proportion in future generations.

Explanation:

Some of the insects in the insect population have one or more genes that impart Bt resistance to them. These insects are able to survive in the presence of Bt toxin while the ones that lack these genes simply die by Bt toxin. Natural selection allows these insects to leave more progeny in the future generation which in turn results in increased frequency of the Bt resistance gene in the future populations. This results in the spread of Bt resistance among insect populations.